The migratory behaviour of wild Atlantic salmon (Salmo salar L.) smolts in the River Test and Southampton Water, southern England

Thirty wild Atlantic salmon Salmo salar (L.) smolts, tagged with miniature acoustic transmitters, were tracked in the River Test and Southampton Water, to describe the freshwater and estuarine patterns of migration. Migration in freshwater was predominantly nocturnal (mean time 23:42) and smolts tagged earlier in the study spent significantly longer in the river before migrating into the estuary than fish tagged later in the study. Movement in the upper estuary section of the River Test was indicative of a nocturnal ebb tide transport pattern of migration. The majority of smolts migrated seawards at night and during an ebb tide (mean time after High Water +1 h 54 min), close to the surface and within the fastest moving section of the water column. Smolts continued to emigrate seawards through the lower estuary and Southampton Water during the ebb tide (mean distance 4908 ± 609 m; mean ground speeds of 35 cm s-1), although the diurnal pattern of movement was not evident. There was also a significant seaward migration of smolts during the latter part of the flood tide (mean distance 3469 ± 522 m; mean ground speeds of 14 cm s-1), suggesting active directed swimming. Smolts emigrated rapidly through Southampton Water, and there was no apparent period of acclimation required when moving from fresh to saltwater. The results are discussed in relation to the environmental cues controlling smolt migration.